On the eve of a major US Senate hearing on the National Security Agency's bulk surveillance, two senators called for major reforms of the NSA's collection of phone records and accused US intelligence leaders of misleading the public about its impact on privacy.

A letter sent by the director of national intelligence, James Clapper, to Ron Wyden, a Democratic senator for Oregon, on Friday said that there had been "a number of compliance problems" with the NSA's bulk, ongoing collection of millions of Americans' phone records, but "no findings of any intentional or bad-faith violations".

On the Senate floor late on Tuesday afternoon, Wyden, a member of the Senate intelligence committee, all but accused Clapper of lying.

Citing classified documents that he did not specify, but referring to "violations of court orders", Wyden said that "these violations are more serious than those stated by the intelligence community, and are troubling". Wyden urged senators to read classified intelligence documents about the bulk surveillance for themselves.

"Any policymaker who simply defers to intelligence officials without asking to see their evidence is making a mistake," Wyden warned.

"The violations I've touched on tonight are a lot more serious than [senators] have been told," Wyden said. "I'm going to keep pressing to make more of these details public."

Wyden's frequent partner on the intelligence committee, Mark Udall, a Democratic senator for Colorado, called on the White House for a "more effective, less intrusive" way to collect phone records related to national security.

"Conduct the program instead through direct queries to phone companies where there is a direct connection to terrorism and espionage," Udall urged on the Senate floor.

Like Wyden, Udall criticised intelligence leaders and their political allies for saying the bulk phone-records collection had helped to stop terrorists.

"I have seen no evidence that the bulk phone-records collection alone played a meaningful role, or any role, in disrupting terrorist plots," Udall said.

It may be "more convenient" for the NSA to conduct a phone-records dragnet, Udall said, but "convenience alone cannot justify" the effort.

Udall also briefly said that the NSA phone-records programme includes the "who, what, where, when and how long" of Americans' phone calls – seemingly a reference to NSA tracking location data on phone calls, which a number of intelligence officials, including Clapper in his letter, have said they do not perform "under this programme".

"Information on whom [Americans] call, when they call, and where they call from is private information," Udall said.

A Udall spokesman clarified that the Senator was not talking about cellular-site tracking, but more about landline location.

"Make no mistake about it … we are going to stay at this," Wyden said.

On Wednesday morning, the Senate judiciary committee will hold its first hearing into the NSA programmes, with anticipated testimony from senior officials from the justice department and NSA. The committee is chaired by Senator Patrick Leahy (Democrat, Vermont) who, alone among committee chairmen in the Senate and House, is vocally in favour of shutting the bulk collection down. Ahead of the hearing, the Obama administration is considering declassifying more secret documents from the Fisa court explaining how the bulk collection is conducted. The Los Angeles Times reported that one of those documents under consideration is a 2007 Fisa court order governing the rules for bulk phone-records collection.